Inductively coupled plasmas are well known as sources to excite and/or ionise sample material in order to analyse the composition of a sample by mass spectrometry (ICP-MS) or optical emission spectrometry (ICP-OES).
U.S. Pat. Nos. 4,682,026 and 4,551,609 show typical inductively coupled plasma sources. A plasma is formed in a gas by the use of a RF current driven in a coil. The gas is confined in a torch that passes through the coil. Sample material is introduced into the plasma through an inner tube in the torch, the material being carried by a stream of carrier gas.
Ions or photons from the plasma are sampled through an aperture in a sampling plate or sampling cone. To be able to detect very low concentrations of analyte species, the aperture should be well aligned with that part of the plasma containing the highest proportion of ionised or excited analyte species. In prior art plasma sources, alignment is achieved by moving the RF electronics, the coil they are attached to, and the torch, with respect to the aperture. An example of this type of stage system for moving a source is given in U.S. Pat. No. 5,185,523 for a microwave induced plasma, in which the magnetron and the microwave power source are mounted onto a stage.
This prior art method has associated problems in that the electronics are bulky and heavy. Typically to move the heavy electronics, motion systems have been placed beneath the electronics enclosure, coil and torch, so as to avoid the use of cantilevers. They are then difficult to access for maintenance, and prone to contact with acidic sample solutions if spillages occur. Such motion systems are also relatively costly. To minimise stray RF emissions from the source, which can adversely affect other instrumentation, carefully placed, secure electrical earths are required. Such earths are more complex and less reliable when the components to be earthed must move. It also is desirable to fix the coil to the electronics as there must be a good degree of impedance matching for efficient transfer of up to 2 kW of power into the plasma. In some prior art alignment systems the electronics have been fixed with respect to the sampling aperture and adjustment effected by moving the coil and torch, but this approach requires a wider range of impedance matching, is less reliable and is costly.
In view of the foregoing discussion, there is a need for an improved inductively coupled plasma alignment apparatus.